Food manufacture is typically characterised by the need to process and treat highly viscous products, e.g. concentrates for carbonated beverages, juices, soups, dairy products and other products of fluid consistency. For natural reasons, the hygiene aspirations and expectations in this context are extremely high to enable the requirements of various authorities to be met.
Plate heat exchangers are used in the industry for a number of different purposes. One problem in using plate heat exchangers for e.g. the food industry is that some products contain fibres and other solid materials mixed in the fluid. In most plate heat exchangers, the heat exchanger comprises one type of plate, which is mounted with every other plate rotated 180 degrees to form two different channels for the fluids, one channel for the cooling medium and one channel for the product that is to be cooled. Between each plate is a sealing provided. Such an arrangement is cost-effective and works for many applications, but shows some drawbacks when it comes to beverages and other products that comprises fibres and other solid materials, since the plates will bear on each other at some contact points. Each plate is provided with ridges and valleys in order to on one hand provide a mechanical stiffness and on the other hand to improve the heat exchange to the liquid. The plates will bear on each other where the patterns of the plates meet each other, which will improve the mechanical stiffness of the plate package. This is important especially when the fluids have different pressures. A drawback of the plates bearing on each other is that each bearing point will constitute a flow restriction where material contained in the liquid may be trapped and can accumulate. The accumulated material will restrict the flow further, causing more material to accumulate. This will somewhat resemble the formation of a river delta, where a small flow difference will deposit some material which in turn causes more material to deposit.
One solution to the problem with clogging of material in a plate heat exchanger is to use a heat exchanger where the product passage is contact-free. This type of heat exchanger reduces the accumulation of material in the product passage. In order to obtain a sufficient rigidity when using traditionally heat exchanger plates for a contact-less plate heat exchanger, the plates are permanently joined together in pairs, e.g. by welding or brazing. In this way, two plates form a cassette with a plurality of contact points between the plates, where the contact points are joined together as well as the rim of the plate. The cassette will be rigid enough to handle some differences in pressure between the two fluids, thereby enabling the contact-free product passage. Since the heating/cooling fluid passage will be inside the cassette, it is important that the two plates are joined together without any leakage. Especially if the heating/cooling fluid will have a relatively high pressure, even a small leakage will cause problems. It is thus indispensable that a completely airtight joint is achieved around the cassette.
One plate heat exchanger having a contact-free passage is known from JP 2001-272194. In this heat exchanger, two plates of the same type having longitudinal grooves are permanently connected to each other to form a cassette, in which longitudinal passages are formed for the heat exchange fluid. Such cassettes are stacked using gaskets, thereby forming a contact-free product passage between two cassettes.
Another heat exchanger having a contact-free product passage is disclosed in WO 2006/080874. In the disclosed heat exchanger, a corrugated and undulating pattern perpendicular to the flow direction is used in order to provide rigidity to the plates and also to improve the heat transfer between the two fluids.
The heat exchanger disclosed in WO 2006/080874 is a so-called semiwelded plate heat exchanger, i.e. a heat exchanger comprising a number of cassettes formed by welding or brazing heat exchanger plates together in pairs. The weld seam normally runs along the side edges of the cassettes and around the portholes. A gasket is disposed between the respective cassettes and is normally made of a rubber material and situated in a groove of the heat exchanger plate. One fluid flows inside the cassettes, and another fluid between the cassettes. The flow passage inside the cassettes is used for the heating/cooling fluid and the flow passage between the cassettes is used for the fibrous fluid. Semiwelded plate heat exchangers tolerate relatively high pressures and make it possible to open the plate package and clean the spaces between pairs of welded heat exchanger plates. The welds which replace the gaskets in every second space between plates round the heat exchange surface of the heat exchanger plates reduce the need for gasket replacement and enhance safety. Tolerances in the plates and/or the joining technique may give an imperfect result or may reduce the maximal allowed pressure in the cassette.
These solutions may function for some applications, but they still show some disadvantages. There is thus room for improvements.